Effects Of Sc On The Microstructure And Mechanical Properties Of Al-6.5Zn-2.3Mg-2.0Cu-0.2Ce-0.1Zr Alloy

Al-Zn-Mg-Cu alloys can be heat-strengthened,under the specific heat treatment state,Al-Zn-Mg-Cu alloys can obtain high strength and high hardness,excellent toughness,form-ability and weld-ability.In addition,Al-Zn-Mg-Cu alloys have replaced steel as the main structure materials in aerospace and transportation fields due to light in weight.So far,the rare earth elements Sc and Zr have been proved the most effective micro-alloying elements in terms of improving the strength,hardness,toughness and corrosion resistance of Al-Zn-Mg-Cu alloys.In this investigation,the effect of Sc on the microstructure,tensile properties and low cycle fatigue properties of Al-6.5Zn-2.3Mg-2.0Cu-0.2Ce-0.1Zr alloy were investigated,in order to provide the theoretical basis for the practical engineering application of this kind of alloy.The results of microstructure observation show that adding Sc can refine the grain size of the cast alloy,as-rolled alloy and T6 stated alloy.A large amount of?'phase and dispersed Al₃?Zr,Sc?phase can be observed in the crystal of Al-6.5Zn-2.3Mg-2.0Cu-0.2Ce-0.1Zr?-0.2Sc?alloy.The discontinuous equilibrium phase precipitates at the grain boundaries,and the precipitate free zones can be seen near the grain boundaries.The results of tensile tests show that adding 0.2%Sc can improve the ultimate tensile strengths,yield strengths and the elongation of Al-6.5Zn-2.3Mg-2.0Cu-0.2Ce-0.1Zr alloy.The tensile strength and yield strength of Al-6.5Zn-2.3Mg-2.0Cu-0.2Ce-0.1Zr?-0.2Sc?alloys increase first and then decrease in the aging time from 10h to 22h,and reach the peak value at 16h,while the elongation of Al-6.5Zn-2.3Mg-2.0Cu-0.2Ce-0.1Zr?-0.2Sc?alloys decrease first and then increase with prolonging the aging time.The morphologies of the tensile fracture surfaces show that Al-6.5Zn-2.3Mg-2.0Cu-0.2Ce-0.1Zr?-0.2Sc?alloys in under aged and over aged state both performing mainly ductile fracture with ductile-brittle mixed fracture.At peak aging time,the method of tensile fracture in Al-6.5Zn-2.3Mg-2.0Cu-0.2Ce-0.1Zr alloy is dominated by brittle fracture,but the method of tensile fracture in Al-6.5Zn-2.3Mg-2.0Cu-0.2Ce-0.1Zr-0.2Sc alloy is dominated by ductile fracture.The results of low-cycle fatigue tests shows that the peak ageing state Al-6.5Zn-2.3Mg-2.0Cu-0.2Ce-0.1Zr?-0.2Sc?alloys exhibit the cyclic stability at the total strain amplitudes from0.4%to 0.8%.The relationship between plastic strain amplitude,elastic strain amplitude and the number of reversals to failure of the peak ageing state Al-6.5Zn-2.3Mg-2.0Cu-0.2Ce-0.1Zr?-0.2Sc?alloys can be described by the Coffin-Manson equation and the Basquin equation,respectively.The morphologies of peak ageing state Al-6.5Zn-2.3Mg-2.0Cu-0.2Ce-0.1Zr?-0.2Sc?alloys'fatigue fracture surfaces show that the fatigue cracks initiate at the free surface of the fatigue samples and propagate in a transgranular mode.